Modern motor vehicles are increasingly using systems which contribute to active or passive protection of the occupants. Systems for tire pressure monitoring protect the vehicle occupants against vehicle damage which is caused, for example, by an abnormal tire air pressure. By way of example, an abnormal tire air pressure can increase the tire wear and the fuel consumption, and can lead to a tire defect (“tire bursting”). Various tire pressure monitoring systems are already known, which either operate on the basis of direct-measuring sensors or identify an abnormal tire pressure by evaluation of the rotation-speed or oscillation characteristics of the vehicle wheels.
DE 100 58 140 A1, which is incorporated by reference, discloses a so-called indirectly measuring tire pressure monitoring system, which detects a tire pressure loss by evaluation of the wheel rotational movement (DDS: Deflation Detection System).
EP 0 578 826 B1, which is incorporated by reference, discloses a tire pressure measurement device which determines a pressure loss in a tire on the basis of tire oscillations, with at least one resonant frequency component being extracted from the tire oscillations.
Patent application DE 10 2005 004 910 A1, which is incorporated by reference, discloses a method for indirect tire pressure monitoring which improves an indirectly measuring tire pressure monitoring system that is based on the evaluation of wheel rotational movement, taking account of the eigen torsion frequency of the tires.
In the case of indirect tire pressure monitoring systems which are known from the prior art, a frequency analysis of the wheel velocity of the individual wheels is frequently carried out in order to identify a wheel-specific reduced air pressure. For this purpose, the torsion eigen oscillation, which is dependent on the air pressure, of the tire is normally used for air pressure monitoring. The eigen frequency or resonant frequency of the torsional oscillation between the rim and the radial belt varies as a result of a pressure loss, because the torsional spring constant between the radial belt and the rim decreases when the tire pressure is reduced. Any pressure loss is therefore deduced from a change/reduction in an eigen or resonant frequency, and an appropriate warning is emitted. However, the eigen frequency or resonant frequency varies not only as a result of a change in the tire pressure but also as a result of further disturbance variables. If these influences are ignored, this can lead to spurious warnings or else to correct warnings not being produced.